I don't know where you read that information, but it is incorrect. Nobody has told the dad what happened to Haley with the teacher. In some of the routes the dad did discover that Haley was in therapy, but he never was told what it was for. It seems to me that the dad thinks that Haley's therapy has something to do with her brother, the MC.
The mother was the one who found out about it when it was happening, and stopped it. But Haley pleaded with her not to do anything about it, because Haley said it was all her own fault, which of course was a delusion that Haley carried with her until she agreed to see the therapist.